A braking system of the kind set forth is disclosed in WO-A-93/00236. In this system the pedal-operated master cylinder communicates with a brake through a pilot pressure operated isolation valve which is spring urged into an open position, and the output from an hydraulic pump is communicated to the isolation valve. When the pressure generated by the pump in response to an electric signal from an electrical sensor associated with the pedal is greater than the force needed to overcome the spring force of the isolation valve, the valve switches to communicate the pump directly to the brake, and isolates the master cylinder from the brake. After the isolation valve changes state the master cylinder is not used to pressurise the brake. However, in the event of malfunction of the electro-hydraulic portion of the system the brake can be applied directly from the master cylinder through the isolation valve in the "push through" mode with the valve switching to its original state.